The second edition of Gas Insulated Substations (GIS) is an all-inclusive reference guide to gas insulated substations (GIS) and its advanced technologies. Updated to the latest technical developments and applications, the guide covers basic physics of gas insulated systems, SF6 insulating gas and its alternatives, safety aspects and factors to choose GIS. GIS technology, its modular structure, control and monitoring systems, testing, installation rules and guidelines for operation, specification, and maintenance. Detailed information on various types for GIS, with 14 reference project explanations and three extensive case studies give information for the best solutions of practical applications. Special solutions using mobile substations concepts, mixed technology switchgear (MTS) with air and gas insulated technology, underground substations, and the use of special GIS substation buildings e.g., shopping centers, parking lots, city parks, business complexes’ or subway stations are explained. Future developments of GIS technology are shown for the next steps in alternatives to SF6, low power instrument transformers, and digitalization of substations.